This invention relates to a sensor, and more particularly to a LIDAR using switchable gratings.
LIDAR is a remote-sensing technology that creates a 3D map of an environment by illuminating a target with a pulsed angularly-scanned laser and analyzing the reflected “point cloud”. The advantages of LIDAR over cameras are well known. Since LIDAR uses emitted light, it is robust against interference from ambient light and has much higher resolution than radar. Artificial light sources are required for nighttime operation. Current computer vision is inadequate for complex scene representation and is susceptible to illumination variation. Currently, there is growing interest in LIDAR systems for a range of platforms including: cars (for applications such as collision avoidance and cruise control systems), robot vehicle, UAVs and wearable displays for night vision. The increasing use of key-hole procedures in surgery is also stimulating medical applications. An exemplary car LIDAR specification (based on the Velodyne® HDL64E LIDAR, manufactured by Velodyne®) has a FOV of 360° in azimuth; 26.5° elevation (+2° to −24.5°); a refresh rate of 15 Hz; a point cloud rate of 1 million points per second; a maximum range of 120 metres; a horizontal resolution of 0.05 degree; a distance error of less than 1 inch; a laser pulse duration of 5 nanoseconds and a power output of typically 60 watts. However, LIDAR equipment meeting this specification is extremely bulk and expensive. LIDAR operating at around 1.55 microns has the advantage of being eye safe with longer range capability but is even more expensive. Most current LIDAR equipment relies on bulky rotating optics technology. LIDAR systems based on imaging array technology have been developed but are currently very expensive. The inventors believe that the key to overcoming the problems of bulk and cost is waveguide optics based on switchable grating technology.
One important class of switchable gratings is based on Switchable Bragg Gratings (SBGs). SBGs are fabricated by first placing a thin film of a mixture of photopolymerizable monomers and liquid crystal material between parallel glass plates. One or both glass plates support electrodes, typically transparent indium tin oxide films, for applying an electric field across the film. A volume phase grating is then recorded by illuminating the liquid material (often referred to as the syrup) with two mutually coherent laser beams, which interfere to form a slanted fringe grating structure. During the recording process, the monomers polymerize and the mixture undergoes a phase separation, creating regions densely populated by liquid crystal micro-droplets, interspersed with regions of clear polymer. The alternating liquid crystal-rich and liquid crystal-depleted regions form the fringe planes of the grating. The resulting volume phase grating can exhibit very high diffraction efficiency, which may be controlled by the magnitude of the electric field applied across the film. When an electric field is applied to the grating via transparent electrodes, the natural orientation of the LC droplets is changed causing the refractive index modulation of the fringes to reduce and the hologram diffraction efficiency to drop to very low levels. Typically, SBG Elements are switched clear in 30 μs. With a longer relaxation time to switch ON. Note that the diffraction efficiency of the device can be adjusted, by means of the applied voltage, over a continuous range. The device exhibits near 100% efficiency with no voltage applied and essentially zero efficiency with a sufficiently high voltage applied. In certain types of HPDLC devices magnetic fields may be used to control the LC orientation. In certain types of HPDLC phase separation of the LC material from the polymer may be accomplished to such a degree that no discernible droplet structure results.
SBGs may be used to provide transmission or reflection gratings for free space applications. SBGs may be implemented as waveguide devices in which the HPDLC forms either the waveguide core or an evanescently coupled layer in proximity to the waveguide. The parallel glass plates used to form the HPDLC cell provide a total internal reflection (TIR) light guiding structure. Light is “coupled” out of the SBG when the switchable grating diffracts the light at an angle beyond the TIR condition. Waveguides are currently of interest in a range of display and sensor applications. Although much of the earlier work on HPDLC has been directed at reflection, holograms transmission devices are proving to be much more versatile as optical system building blocks. Typically, the HPDLC used in SBGs comprise liquid crystal (LC), monomers, photoinitiator dyes, and coinitiators. The mixture frequently includes a surfactant. The patent and scientific literature contains many examples of material systems and processes that may be used to fabricate SBGs. Two fundamental patents are: U.S. Pat. No. 5,942,157 by Sutherland, and U.S. Pat. No. 5,751,452 by Tanaka et al. Both filings describe monomer and liquid crystal material combinations suitable for fabricating SBG devices. One of the known attributes of transmission SBGs is that the LC molecules tend to align normal to the grating fringe planes. The effect of the LC molecule alignment is that transmission SBGs efficiently diffract P polarized light (ie light with the polarization vector in the plane of incidence) but have nearly zero diffraction efficiency for S polarized light (ie light with the polarization vector normal to the plane of incidence. Transmission SBGs may not be used at near-grazing incidence as the diffraction efficiency of any grating for P polarization falls to zero when the included angle between the incident and reflected light is small.
There is a requirement for a compact, lightweight, low cost LIDAR capable of providing wide angle, high-resolution, long-range operation.